Overheat protector for heated windshields



Aug 8, 1950 w. B. BREWER 2,518,277

OVERHEAT PROTECTOR FOR HEATED WINDSHIELDS Filed Aug. 6, 1947 W if i4 wmosmELo l5 coNTRoL UNIT iNvENToR WAR D B. BREwR Agent developed in the convolute of wire.

Patented Aug. 8, v1950 OVER/HEAT PROTECTGR- FOR HEATED WINDSHIELDS Ward B. Brewer, Glendale, Calif., assigner 'to Lockheed Aircraft. Corporation,

Calif.

Burbank,

Application August 5, 1947, Serial N e. llll (Cl. 21S- 19) l lClaim.

This invention relates to preventing inadvertent overheating of windshields and the like which are provided with heating means for preventing the formation or removing accumulations cf ice thereon. Windshields, particularly for aircraft, are subject to ice accumulations both on the ground and in night and heating thereof, to melt the ice, is desirable practice. However, such wind- ,shields are normally made as glass sandwiches bonded together with a tough thermoplastic resin interlayer of the acrylate type. Because the preferable types of interlayer materials are thermoplastic, inadvertent overheating of such laminated windshields is apt to cause localized melting or bubbling of the interlayer material, permanently destroying the optical usefulness thereof. Heating methods heretofore in use inB clude hot air distributed over the inner surface or between spaced layers of a double glazed windshield, wires embedded in the interlayer material and heated by an electric current, and electrically conductive thin transparent coatings on the glass that for my purposes may be considered as equivalent to an innite number of fine wires overlying or embedded in the glass laminate.

It has also been proposed to embed a convolute of wire in such glass laminates, which wire is incorporated as one arm of a bridge circuit in an attempt to measure the temperature The various disadvantages of such an arrangement include serious manufacturing problems encountered in YVplacing such a wire insert in an exposed of the glass laminate, involves the use of delicate electrical measuring instruments in the bridge circuit, the wire can not be replaced, repaired, or adjusted when once built into the glass laminate,

and the wire must be tailored to each individual windshield location, size, heating wire pattern, and exposure, since it must be .electrically insulated from the heating wires and responds only v-to the heat transmitted thereto through the glass laminate from the localized hot spots created by the heating wires.

It is known that the embedded heating wires Such softening or bubbling permanently ruins the entire Windshield since the optical distortions produced thereby inter- 'fere with the pilots vision and at the least render the otherwise invisible heating Wires very prominent in the eld of vision. This maximum temr perature of 200 F. may occur before the surface temperature of the glass panel reaches a temperature of F. It will, therefore, be evident that to inadvertently turn on the heat when the Windshield temperature is much above the freezing point risks rather prompt and lasting damage to the windshield. It is accordingly an object of this invention to provide an improved and simplied control for a windshield heating system of the class described wherein the control responds to a predetermined exposed surface temperature in the order of 40 F. to 50 F. to prevent operation of the heating system above such surface temperatures.

It Will be understood in this connection that the heat dissipation from the exposed surface of the windshield will vary over wide limits, from a loss in flight to a low loss or even gain on the ground, so that it is a further object of this invention to provide a control of the type described that will embody a thermal capacity analogous to that of the windshield to be protected so that when the control unit is exposed to approximately the same type of air flow and atmospheric conditions as the controlled equipment, a thermostat will open a heater circuit at a safe temperature and will reclose the circuit when icing conditions again appear.

lt is also an object of this invention to provide a. protector or control unit for heated windshields or the like that can be mounted and exposed separately from the windshield and will there fore be replaceable and adjustable Without disturbing or replacing the Windshield itself.

It is also an object of this invention to provide a protector or control unit of the type and for the purposes described that can be used to control various similar types of windshield heaters on different airplanes without requiring the tailoring of the control unit to each specific installation.

Other objects and features of the invention will be readily understood from the following detailed description of typical preferred forms of the invention wherein reference will be made to the accompanying drawings in which:

I have chosen to illustrate the use of my invention in connection with an airplane windshield heated by means of embedded Wires, as shown in Figure 1;

Figure 2 is an enlarged section in diagrammatic form of my protector unit as mounted in the skin of an airplane: and

Figure 3 is a wiring diagram of the heating and protecting circuits of my invention.

Where windshield deicing arrangements are used in airplanes, it is customary to confine such equipment to forwardly facing panels as indicated at I in Figure 1, where a portion of an airplane fuselage Il is shown. It is not so essential to heat the side windows I2 although they can also be heated if desired. While the Windshields may be heated in a number of ways, the present invention relates to a `protective control of such heating and I have chosen to illustrate my con trol as applied to a windshield having a plurality ofvne heating wires I3 embedded in the interlayer of the laminated glass panel. These Vheating wires I3 are arranged in parallel and are connected in parallel to common electrical connections I 4 and I5 also embedded in the inter-1 layer at the margins of the glass panel.. As here tofore used these connections form a circuit through a manual control switch I5 and the electrical system of the airplane as represented by a vbattery I1.

.My invention contemplates the insertion in the aforesaid circuitl .ci a control unitg-enerally indicated by A in Figure 1. This control unit is .de- `siralcly mounted in the skin 5S of the airplane in a location exposed to ksuhstantially the` same type of airow and. atmospheric lcrmditions as the 4windshields which it controls, it being' understood that the unit may simultaneously control one or more separate windshieldy panels, as the conditions cf the particular airplane installation .may reduire.

The unit A comprises a Well-like chamber resembling an inverted cup i9, the outer surface oi the bottom 2u of which is mounted 4flush with of temperature of the .exposed .surface of the control unit is the .same as that. of the. windshield surface to be. protected. The heater vcoil 23 is also chosen to match the thermal capacity of .the control surface, so that these two variables are .dependent on eachother, and 'a wide .range of combinations are possible, the thermal capacity fheing increased or decreased .as a function of the heat delivery from the heater coil 23 in orden to` produce an overall result matching the rate of temperature rise in the windshield surface. The latter variable is dependent on variations in glassfand interlayer materials and thickness,I .and

the electrical energy .supplied Vto the heating Wires .I3 embedded therein.

The heater coil 23. is .desirably cemented in place against the inside of the cup bottom and is overlaid with heat insulation 24 to prevent ldissjpation of heat to the interior of the cup.

.The heater coil 23 is connected in vparallel with Vthe windshield heating wires I3 as shown in Figure and a pilots indicating lamp 25 may also be connected in parallel with both heaters to warn the pilot that the 'heating circuit is closed.

An adjustable` thermostatc switch 2t is mounted in .a .container 2i in heat transferring relationship to the walls of the cup IS and connected in series-with the manual switch I6. Thus when the temperature of the cup walls reaches a chosen maximum Ythe thermostatic vswitch opens to open the heating circuit, and when the switch `255 closesdue to `a drop in tem- `heating .circuit under such conditions.

perature it energizes both heating systems if the manual switch I5 is closed.

In practice, I have found that windshield heating is desirable as the outer surface temperature of the windshield drops to approximately F. Commercially Vavailable thermostatic switches have some spread between the opening and closing temperatures. For example the Paul Henry Company, Model C-Sl thermoswitch that I have used, if set to close at a surface temperature of 35 will not open until the surface tempera ture rises to approximately 47 F., a spread of approximately 12 F. This spread is well Within the permissible limits, as a surface temperature in the windshield of 47 F. will not result in damage to the interlayer, as I have found that the heating wires I3 can produce a surface temperature of approximately 86 F. on the ground Without localized overheating of the interlayer adjacent the wires i3. VSince it is obviously very easy to attain such va surface temperature in still air the use of a thermostat cutting out below l?. prevents turning on the heat under ground conditions wherein the windshield is already so Warm that any added heat would quickly damage the windshield interlayer in the event' a high ambient air temperature prevented rapid dis.- spation of such heat..

It should he noted in this connectionthat under ground or still air conditions the .heat transmission or heat dissipation from the sul."s

.faces of the windshield and control unit lrepresent av low-loss condition wherein the rate. of temperature rise is determined by the heat capacity of the windshield and the control unit. In cold cli'- mates an airplane may gather a coating of .ice .or snow while standing .and it will be desirable to melt such coating by the operation of the. heating system. If the heat capacity of the control unit is approximately matched to that of the Windshield controlled thereby the thermostatic switch 2e can adequately and safely Qontrol the even though the embedded wire types of heated windshields have very little margin available to withstand localized overheating adjacent the wires. Under such static conditions the exact location and exposure .of the control unit is not so critical as is the case where high-loss conditions result from flight under icingatmospheric Conditions. High loss or rapid .heat .dissipation thenv results from the high air velocities flowing over the Windshield and control url.itrsurface, so that. the control unit location should he chosenr to expose it to substantially the same type of airowand atmospheric conditions as the. windshieldwhicn it controls..

By Way of example, an embedded wire type of windshield was .arranged to receive heat at the rate of two Watts per square inch of surface. The resistance of the control unit heater wire 2'3 was so chosen that under steady state conditions heat would he supplied to the exposed bottom 2l) of the cup .at the same yrate as heat. reached. the surface of the windshield. .In the specific .eX- ample the heater 123 was. rated `at '5,5 watts. Under these condtions, and with equal exposure, the surface temperature of the unit and of the windshield will be the same, and with equivalent heat capacity Vin the windshield and .controlunit the rate of-temperature rise of the control unit will be the same as that in the windshield, so that the thermostat switch 2G can beset to close the heating circuits `when the surface temperature of the control unit falls to the ic-ing temperature of 35 F. The actual operating temperatures at the thermostat itself will naturally be higher than at the exposed surface but the difference will be a function of the heat capacity and heat dissipation from the surface of the control unit, which factors match those of the windshield to be protected.

It will thus be seen that I have provided a windshield heater protector that can be remotely disposed relative to the windshield to be protected, yet that will have the same initial rate of rise of surface temperature as the windshield and that will have heat supplied to the surface of the protector at the same rate as it reaches the surface of the windshield; thus producing comparable temperatures in the protector and windshield. Y

Having thus described my invention and the present preferred embodiments thereof, I vdesire tofemphasize the fact that many modifications may be resorted to in a manner limited only by a just interpretation of the following claim;

claim:

fAn overheat protector for electrically heated windshields and the like, comprising a remotely positioned protector unit, said unit including an inverted cuplike member, the bottom ofnwhich is exposed to substantially the same typeY of airflow and atmospheric conditions as the windshield Which it protects, said cup-like member having a thermal capacity such that the rate of temperature change in the exposed part of said member substantially matches that of `ithe Wlnd shield to be protected, heating means in said cup-like member adapted to produce a temperature rate of changefin said member substantially matching that in fsaid windshield, a thermostatic switch in said cuplike member, and ajeircuit including said windshield and heating' means with said switch whereby said switch controlsboth the windshield andthe heating means in'response to varying temperatures in said cup-lik'e'r'nember.

WARD B. BREWER.

REFERENCES CITED The following' references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,781,244 Y Oswald N ov. 11, 1930 1,900,831 i Marter Mar. '1, 1933 2,006,006 y ,Zaiger June 25, 1935 2,065,844 y Wattles Dec. 29, 1936 2,146,604 l Taylor Feb. 7, 1939 2,195,958 Kearsley Apr. 2, 1940 2,429,420 McMaster Oct. 21, 1947 FOREIGN PATENTS Number Country Date 221,480 Great Britain Feb. 19, 1925 

